/*
 * Copyright 2002-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.core.convert.support;

import org.springframework.core.convert.ConversionService;
import org.springframework.core.convert.TypeDescriptor;
import org.springframework.core.convert.converter.ConditionalGenericConverter;
import org.springframework.lang.Nullable;

import java.util.Collection;
import java.util.Collections;
import java.util.Set;

/**
 * Converts a Collection to an Object by returning the first collection element after converting it to the desired targetType.
 *
 * @author Keith Donald
 * @since 3.0
 */
final class CollectionToObjectConverter implements ConditionalGenericConverter
{

    private final ConversionService conversionService;

    public CollectionToObjectConverter(ConversionService conversionService)
    {
        this.conversionService = conversionService;
    }

    @Override
    public Set<ConvertiblePair> getConvertibleTypes()
    {
        return Collections.singleton(new ConvertiblePair(Collection.class, Object.class));
    }

    @Override
    public boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType)
    {
        return ConversionUtils.canConvertElements(sourceType.getElementTypeDescriptor(), targetType, this.conversionService);
    }

    @Override
    @Nullable
    public Object convert(@Nullable Object source, TypeDescriptor sourceType, TypeDescriptor targetType)
    {
        if (source == null)
        {
            return null;
        }
        if (sourceType.isAssignableTo(targetType))
        {
            return source;
        }
        Collection<?> sourceCollection = (Collection<?>) source;
        if (sourceCollection.isEmpty())
        {
            return null;
        }
        Object firstElement = sourceCollection.iterator().next();
        return this.conversionService.convert(firstElement, sourceType.elementTypeDescriptor(firstElement), targetType);
    }

}
